Papillomaviruses (PV's) play a critical role in inducing both benign and malignant tumors in humans. The development of these neoplasms appears to result from the complex interaction of several viral oncoproteins with cell regulatory proteins, including tumor suppressor proteins. The PV E5 oncoproteins constitute a unique class of transforming proteins; they are small, hydrophobic, membrane-embedded polypeptides which appear to perturb cellular signal transduction pathways. The best characterized E5 oncoprotein, that of bovine papillomavirus type 1 (BPV-1), activates the platelet derived growth factor receptor (PDGF-R) in fibroblasts and binds to two cellular proteins in-vivo: PDGF-R and the 16K V-ATPase protein. We have recently shown that all three of these proteins (E5, 16K, and PDGF-R) are present in a ternary complex whose formation appears to be dependent upon the E5 transmembrane region (TMR). We propose to investigate the molecular mechanism by which BPV E5 forms homodimers and interacts with its cellular target proteins, with a specific focus on the important role of TMR helical interactions. Our studies will generate and analyze a series of mutants of the BPV E5 TMR for their ability to form cysteine-independent dimers, bind PDGF-R and 16K (in-vivo and in-vitro), and transform cells. We will also investigate the role of TMR dimerization in receptor activation and the necessity of Golgi localization for cell transformation. In addition, we will also place emphasis on the HPV E5 proteins. Using a quantitative keratinocyte assay developed in our laboratory, we show in Preliminary Studies that E5 enhances the transforming activity of the HPV16 E6/E7 genes. We will use this assay to genetically dissect the HPV E5 proteins and establish correlations between E5 biological activity and ability to bind 16K and the EGF receptor. Finally, we will study the role of the 16K protein in both BPV and HPV-mediated cell proliferation. The recruitment of 16K into BPV E5-activated receptor complexes suggests an important role for this protein (and potentially the V-ATPase) in receptor signalling and/or processing. In summary, our studies are designed to understand how PV E5 proteins (and their cellular target proteins) regulate cell proliferation and contribute to neoplasia.